Recently, dimensions of wireless devices and mobile devices such as a mobile terminal are minimized. The mobile terminal is suitably used for a keyless entry system in a vehicle. Thus, it is required to reduce dimensions of an antenna mounted in the mobile terminal.
The above built-in antenna apparatus in the mobile terminal and the like is described in JP-A-2000-244219 (corresponding to EP-1154513). The antenna apparatus is built in a wireless communication terminal for communication, and includes a loop antenna device having a loop surface, which is perpendicular to a substrate.
Since the loop antenna device is perpendicular to the substrate, the loop surface is perpendicular to a body of a person when the person as a user uses the wireless communication terminal to communicate. Thus, a gain of the loop antenna device is improved.
Further, in the antenna apparatus, an antenna element having a loop shape for providing the loop antenna device is bent at a midpoint of the element by 90 degrees so that the antenna element can receive both polarized waves of a vertical polarized wave and a horizontal polarized wave.
In the antenna apparatus, a balance-unbalance conversion circuit is coupled with a power supply point of the loop antenna device. The balance-unbalance conversion circuit reduces current flowing into the substrate having a ground potential. Thus, the balance-unbalance conversion circuit prevents the substrate from functioning as an antenna. If the substrate functions as the antenna, when the body of the user approaches the antenna apparatus, the function of antenna in the apparatus is reduced by influence of the human body. Thus, in JP-A-2000-244219, the balance-unbalance conversion circuit reduces the current to the substrate, so that the substrate does not function as the antenna. Thus, the reduction of gain of the antenna apparatus caused by approach of the human body is restricted.
In the antenna apparatus, since the substrate does not function as the antenna, a polarized wave ratio between the vertical polarized wave and the horizontal polarized wave is secured. Specifically, the polarized wave ratio is substantially constant even when conductive object such as a human body approaches the antenna apparatus.
However, in the antenna apparatus, although the polarized wave ratio is constant without depending on existence of a conductive object, the constant polarized wave ratio is performed with using the balance-unbalance conversion circuit for reducing current to the substrate so as not to form the antenna with the substrate. Thus, it is necessary to use the balance-unbalance conversion circuit. Thus, a manufacturing cost of the antenna apparatus increases.
Further, only the loop antenna device provides to receive both of the vertical polarized wave and the horizontal polarized wave. Thus, it is difficult to obtain high gain with respect to a whole of the antenna apparatus.